FIG. 1 illustrates schematically a pixel PXi of the pixel matrix of one such back side illumination image sensor CPT.
More precisely, the pixel PXi is produced in a semiconductor substrate SB, for example made of p−-doped p-type silicon. The photosensitive region of the pixel is delimited by lateral insulation regions RIS. For example, these lateral insulations can be made up of trenches of silicon oxide, or else by more heavily p-doped regions. The photosensitive region itself includes domains of n-, p−- and p+-type conductivity (p+ signifying a heavier doping than p). These doping regions are arranged in the aim of forming a photodiode, for example of n-i-p type, serving to capture, then store the photogenerated charges. The peripheral doping regions serve to create an electric field gradient, to better drain the photogenerated charges towards the photodiode.
Above the front side FAV of the substrate, the integrated circuit conventionally includes a first part 1, commonly named FEOL (front end of line) by those skilled in the art, including the components of the integrated circuit such as transistors. In this respect, the pixel PXi includes the transfer gate TG of a transfer transistor in this part 1.
Part 1 of the integrated circuit CI is conventionally overlaid by an interconnect part 2, commonly named BEOL (back end of line) by those skilled in the art, notably including various metallization levels intended to mutually interconnect the various components of the integrated circuit and with connection pads situated on the front side of this assembly.
Above the back side FAR of the substrate is found a stack of layers including a first layer 3, made of silicon dioxide, overlaid by an anti-reflective layer of silicon nitride Si3N4 itself overlaid by another layer 5 of silicon dioxide. This layer 5 is itself supported by a part 6 notably including colored filters as well as back side contact pads.
The sensor CPT is a back side illumination sensor because the rays of light penetrate the pixel PXi via the back side FAR of the substrate.
The silicon nitride layer 4 is an optical anti-reflection layer, the aim of which is notably to effect an adaptation of the optical index between the outside environment and the photodiode in such a way as to avoid reflection of the rays of light coming from the outside environment.
In theory, when the sensor is not illuminated, the photodiode does not produce any current and the resulting image is perfectly black. In practice, there is always a slight dark current that attempts are made to minimize. And such a back side image sensor exhibits dark current degradation.
The degradation comes from plasma-based treatments used during the treatments of the back side of the sensor, particularly for forming the back side metallic contact pads, of aluminum for example, and for forming the layer protecting the colored filters, which aims to protect these filters during the slicing of the sensors along the slicing lines of the semiconductor wafer.
More precisely, the plasma charges the silicon oxide-silicon nitride stack triggering a trapping of positive charges (holes) in the anti-reflective silicon nitride layer, holes which will then migrate to the interface with the substrate to create a dark current by recombination with electrons.